1. Field of the Invention
The present invention relates to a data transmission system for transmitting various kinds of data respectively to receiving stations which are operable to receive only a respective one of the data.
2. Description of the Prior Art
The conventional data transmission system is shown in FIG. 1.
In FIG. 1, the reference numerals (1a)-(1n) designate transmission stations; (2x), (2y) and (2z), respectively, designate receiving stations for receiving data X, Y and Z; and (3) designates an exchange network. Each transmission station (1a)-(1n) comprises a network controller (11), a modulator (12), an encoder (13), a scanner (14), a demodulator (15), a decoder (16) and a hybrid coil (17). Each receiving station (2x), (2y), and (2z) comprises a network controller (21), a demodulator (25), a decoder (26), a modulator (22), an encoder (23) and a hybrid coil (27).
The operation of the conventional system is nextly illustrated.
In FIG. 1, the receiving station (2x) selectively receives only the data X among the data transmitted from the transmission stations (1a)-(1n) and the receiving stations (2y) and (2z) respectively and selectively receive only the data Y or Z among the data transmitted from the transmission stations.
Referring to FIG. 3, the operation of the known system is illustrated. The sequential operations (1)-(8) shown in FIG. 3 are nextly discussed.
The transmission passage between the receiving station (2x) and the transmission station (1a) is formed as follows.
(1) A telephone number of the transmission station (1a) is transmitted by dialing from the network control unit (21) in the receiving station (2x) to the exchange network (3). (calling)
(2) At the exchange network (3), a ringing such as bell ringing or tone is given to the network control unit (11) in the transmission station (1a) by the received telephone number. (ringing)
(3) At the ringed transmission station (1a), the DC loop of the line in the network control unit (referring to as NCU) (11) is closed in response to the ringing from the exchange network (3).
(4) At the exchange network (3), the response of NCU (11) is received to inform the fact of the connection of the line of the transmission station (1a) to NCU (21) by a stopping of ring back tone or a reversal of line polarity. The ring back tone is a signal for informing the fact of ringing at the called station to the calling station.
The transmission line between the receiving station (2x) and the transmission station (1a) is formed by the operation (1) to (4). The sequences of (1) to (4) are substantially the same as the connection of the usual telephone. In the usual telephone, the operations of the terminals are manual operations. In the data transmission system, the connection controls at the terminals are carried out through the network control unit (11) or (21) in automatic or semi-automatic operation.
The data transmission sequences between the receiving station (2x) and the transmission station (1a) are nextly illustrated.
(5) The data X demand signal at the receiving station (2x) is input into the encoder (23), in which it is converted into the data X demand code (series pulse code for demanding transmission of data X). The series pulse code is converted into FS signal by FSK modulator (22) to transmit the FS signal to the transmission line.
(6) The data X demand FS signal transmitted into the transmission line, is demodulated by the FSK demodulator (15) on the side of the transmission station (1a) and converted into a data X demand code (series pulse code). The resulting data X demand code is converted into the data X demand signal by the decoder (16) for application to the next scanner (14).
(7) The scanner (14) discriminates kinds of data by the data demand signal from the receiving station side. Only the designated data are transmitted, after scanning, to the encoder (13) in the transmission station side.
In the above embodiment, only data X are input to the encoder (13) by the scanner. At the encoder (13), the input data X signal is converted into a series pulse code for application to the next FSK modulator (12). At the FSK modulator (12), the series pulse code corresponding to the data X is converted into a FS signal to transmit it to the transmission line. (The operations of the encoder and the FSK modulator is the same as that of the sequence (5).)
(8) The FS signal corresponding to the data X transmitted to the transmission line is demodulated by the FSK demodulator (25) in the receiving station (2x) side for conversion into the series pulse code for the data X. Then, the data X as a parallel signal is reproduced by the decoder (26) at the receiving station side.
As described, when the data X is reproduced at the receiving station (2x) side, NCU (21) in the receiving station (2x) is to open the DC loop of the line to the exchange network (3) to disconnect the terminal from the exchange network. On the other hand, the disconnection of the exchange network from the transmission station is performed by NCU (11) under the condition of the completion of the data transmission at the transmission station.
The operation for reproducing the data X in the transmission station (1a), into the receiving station (2x) by the demand of the receiving station (2x) has been illustrated. The data X in the transmission stations (1b) . . . (1n) can be collected into the receiving station (2x) by the same manner. In this case, each telephone number in the sequence (1) should be changed corresponding to each transmission station. The collections of the data Y and data Z from the receiving station (2y) or (2z) can be attained by the same manner. In this case, the data demand signal is changed to the data Y demand signal and the data Z demand signal.
The receiving station (2x) selects the data X transmitted from the transmission station (1a) by the foregoing steps. The data X transmitted from the other transmission stations (1b)-(1n) are also selected by the same steps.
The receiving stations (2y), (2z) respectively select the data Y or the data Z transmitted from the transmission stations (1a) to (1n) by the same steps.
In the conventional system, each data transmission demand code is transmitted from each of the receiving stations (2x), (2y), (2z) to the transmission stations (1a)-(1n) and the transmission stations (1a)-(1n) transmit only the selected data.
Thus, the receiving stations (2x), (2y), (2z) should have the function of transmitting the data transmission demand code and the transmission stations (1a)-(1n) should have the function of receiving the data transmission demand code. Therefore, hardware for the transmission stations and the receiving stations is increased thereby disadvantageously increasing power consumption, size and cost.
The disadvantages are caused because only one kind of modulation central frequency of the modulator (12) in the transmission stations (1a)-(1n) is given and the modulation is carried out in the same modulation central frequency for all kinds of the data. The modulation central frequency of the receiving station (2x), (2y), (2z) is set to be the same as the modulation frequency of the transmission station. Thus, the received data can not be selected. Therefore, the data transmission demand code for indicating the kind of data is output from the receiving station through the encoder (23) and the modulator (22) to the transmission station and the received data transmission demand code is demodulated and decoded by the demodulator (15) and the decoder (16) in the transmission station so as to control the circuit.